In the catalytically assisted preparation of carbodiimides from aromatic polyisocyanates and in particular from 4,4′-diisocyanatodiphenylmethane (4,4′-MDI), the reactivity of the system can be subject to large fluctuations. This can be attributed at least partly to the content of hydrolyzable chlorine (HC) in the system. These reactivity fluctuations are frequently compensated for by adaptation of the catalyst concentration, with a phospholine oxide being used as catalyst in many cases. Although an increased phospholine oxide concentration influences the reaction rate in the desired way, it leads to a significant decrease in the product stability. The decrease in the product stability is revealed by an increase in the viscosity of the polycarbodiimide-containing composition.
There has in the past been no lack of attempts to reduce the content of hydrolyzable chlorine in the polyisocyanates. U.S. Pat. No. 3,516,950 is concerned with the production of rigid polyisocyanurate foams and in this context states that it is advantageous to use organic polyisocyanates which do not have an excessively high content of hydrolyzable chlorine. In order to reduce the HC content of the crude polyisocyanates, these can be treated with lime or other weakly basic materials. As an alternative, they can be heated to temperatures in the range from 150° C. to 220° C. while a stream of inert gas is passed through the liquid in order to assist the removal of hydrogen chloride. In one example, it is also described how crude MDI is heated to from 180° C. to 190° C. in a stream of nitrogen for 20 hours. It is reported that the NCO content is here reduced from 86.4% to 80.0% of the theoretical NCO content of MDI and the content of hydrolyzable chlorine decreases from 0.4% to 0.17%.
U.S. Pat. No. 4,088,665 relates to the partial carbodiimidization of monoisocyanates, diisocyanates and/or polyisocyanates in the presence of a phosphorus-organic compound specified there, followed by stopping of the carbodiimidization reaction by addition of a carbamoyl chloride or bromide. In one example, it is described how a reaction vessel is charged with 4,4′-MDI and flushed with nitrogen. After flushing, the contents are heated to 65° C. for one hour.
A general overview of the preparation of storage-stable carbodiimides may be found in DE 10 2004 033 849 A1. This patent application discloses processes for preparing organic isocyanates having carbodiimide and/or uretonimine groups by partial carbodiimidization of isocyanate groups using catalysts of the phospholine type, and subsequent stopping of the carbodiimidization reaction by addition of a silylated acid of the formula X—[Si(CH3)3]n, where X in the formula is the neutral acid radical as is obtained by removal of the acidic hydrogen atoms from an n-basic acid having a pKa of not more than 3, with hydrohalic acids being excepted, and n is an integer of 1-3. In addition to the silylated acid, an unsilylated acid and/or an acid chloride and/or a sulfonic ester is added.
US 2005/0282993 A1 describes a process for preparing liquid storage-stable organic isocyanates having carbodiimide and/or uretonimine groups. Here, acidic impurities in the organic isocyanate are neutralized by means of an acid scavenger. A partial carbodiimidization is subsequently carried out in the presence of a catalyst of the phosphorus oxide type, followed by stopping of the carbodiimidization reaction by addition of an acid.
EP 1 971 623 B1 has the object of providing a simple and economical process for producing liquid, storage-stable and light-colored isocyanate mixtures having carbodiimide and/or uretonimine groups. The patent document discloses a process for preparing organic isocyanates having carbodiimide and/or uretonimine groups, in which one or more organic isocyanates having a Hazen color number of <100 APHA, preferably <50 APHA, are partially carbodiimidized using catalysts of the phospholine type and the carbodiimidization reaction is subsequently stopped, with the carbodiimidization being carried out in the presence of a silylated acid amide.
Advantages indicated are that the reactivity of the reaction mixture is increased and/or made uniform by the presence of a silylated acid amide during the carbodiimidization. In this way, the required reaction time is said to be able to be reduced or kept low and/or the required amount of catalyst be reduced. Both the isocyanates containing carbodiimide and/or uretonimine groups and the prepolymers prepared therefrom have, according to this patent document, good storage stability and a light color. The influence of the increased content of hydrolyzable chlorine on the reactivity or the reaction time is discussed with the aid of comparative examples 1 and 2. In examples 1 and 2 according to the patent, an improved reactivity is achieved compared to comparative example 2 and at the same concentration of catalyst used leads to shorter reaction times in examples 1 and 2 according to the patent.
EP 2 371 873 A1 relates to the production of polycarbodiimide and of aqueous dispersions of polycarbodiimide and describes a process for producing a composition comprising polycarbodiimides, which comprises the step of reacting a reaction mixture comprising an aliphatic or cycloaliphatic polyisocyanate and a carbodiimidization catalyst, characterized in that the reaction mixture is maintained at a temperature of from 160° C. to 230° C. during the reaction in the presence of 50-3000 ppm of carbodiimidization catalyst. Application of reduced pressure (500 mbar in the examples) or introduction of nitrogen or a combination of both features is said to be advantageous since the reaction gases are removed thereby and the carbodiimidization reaction is thus accelerated (paragraph [0011], examples). Even though the significantly milder reaction conditions for the reaction of aromatic isocyanates is indicated, it was not to be expected that the storage stability of partially carbodiimidized aromatic polyisocyanates can be improved by applying such a measure to the polyisocyanate before and/or during the reaction thereof.
EP 1 671 988 B1 describes, inter alia, carrying out the carbodiimidization reaction under nitrogen, which a person skilled in the art will understand to mean that the gas space above the liquid reaction mixture was filled up with nitrogen, which is not the same as passing a stream of nitrogen through the polyisocyanate before and/or during the reaction thereof.
WO 2007/076998 A1 discloses a process for preparing organic isocyanates having carbodiimide and/or uretonimine groups, in which one or more organic isocyanates having a Hazen color number of ≤100 APHA, preferably ≤50 APHA, are partially carbodiimidized using catalysts of the phospholine type and the carbodiimidization reaction is subsequently stopped. The carbodiimidization is carried out in the presence of at least one secondary or tertiary amine which contains at least one aliphatic or cycloaliphatic or araliphatic hydrocarbon radical which is optionally substituted and optionally contains heteroatoms. According to this patent application, the use of a secondary or tertiary amine which contains at least one aliphatic or cycloaliphatic or araliphatic hydrocarbon radical which is optionally substituted and optionally contains heteroatoms results in the reactivity of the starting isocyanate being increased.
This could, for example, be caused by the amines countering the reactivity-decreasing effect of potential HCl-eliminating secondary components in the starting isocyanate by acting as bases and binding HCl as hydrochloride. However, other mechanisms of action may also be possible.
As advantages of the process described, it is stated that the reactivity of the reaction mixture was increased and/or made uniform by the presence of a secondary or tertiary amine which contains at least one aliphatic or cycloaliphatic or araliphatic hydrocarbon radical which is optionally substituted and optionally contains heteroatoms during the carbodiimidization. In this way, the required reaction time is said to be able to be decreased or kept low and/or the required amount of catalyst be reduced. Both the isocyanates containing carbodiimide and/or uretonimine groups and the prepolymers prepared therefrom additionally have, according to the statements of this patent application, a good storage stability and a light color.